staging: comedi: rtd520: sample types are unsigned

Sample values in comedi are generally represented as unsigned values.
Use unsigned types for sample value manipulations in the "rtd520" driver
for consistency.

Also replace the hand-coded munging of 2's complement sample values with
calls to `comedi_offset_munge()` and AND with `s->maxdata` to strip off
any extra sign bits.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>